Recovery of gold and/or silver from scrap

ABSTRACT

Gold and silver are recovered selectively such that gold and silver are separated from non-silver and non-gold material within the scrap. Gold and silver are recovered from scrap material using mixtures of acids, in some instances. The mixture comprises nitric acid and at least one supplemental acid, such as sulfuric acid or phosphoric acid. The amount of nitric acid within the mixture are relatively small compared to the amount of sulfuric acid or phosphoric acid within the mixture. The recovery of gold and silver using the acid mixtures are enhanced by transporting an electric current between an electrode and the gold and silver of the scrap material. Acid mixtures are used to recover silver from particular types of scrap materials, such as scrap material comprising silver metal and cadmium oxide and scrap material comprising silver metal and tungsten metal.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/944,885, filed Feb. 26, 2014,and entitled “Recovery of Gold and/or Silver from Scrap,” which isincorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

The disclosure is generally related to the recovery of gold and/orsilver from materials containing gold and/or silver.

SUMMARY

The recovery of gold and/or silver from gold and/or silver containingscrap is generally described. According to certain embodiments, the goldand/or silver can be recovered selectively, such that gold and/or silveris at least partially separated from non-silver and/or non-gold materialwithin the scrap. The subject matter of the present invention involves,in some cases, interrelated products, alternative solutions to aparticular problem, and/or a plurality of different uses of one or moresystems and/or articles.

Certain aspects are related to methods of recovering gold and/or silverfrom a scrap material. In some embodiments, the method comprisesexposing the scrap material comprising the gold and/or silver and atleast one base metal to a mixture comprising nitric acid and at leastone supplemental acid; and recovering at least a portion of the goldand/or silver from the scrap material, wherein the amount of nitric acidwithin the mixture is less than or equal to about 10 wt %.

In some embodiments, the method comprises exposing the scrap materialcomprising the gold and/or silver and at least one base metal to a fluidcomprising an oxidant having the ability to dissolve gold and/or silver;and recovering at least a portion of the gold and/or silver from thescrap material; wherein the amount of the oxidant within the fluid isless than or equal to about 10 wt %.

In certain embodiments, the method comprises exposing the scrap materialcomprising the gold and/or silver and at least one base metal to amixture comprising nitric acid and water; and recovering at least aportion of the gold and/or silver from the scrap material, wherein theamount of water within the mixture is less than or equal to about 17 w%.

In certain embodiments, the method comprises exposing the scrap materialcomprising the gold and/or silver and at least one base metal to amixture comprising sulfuric acid and nitric acid and/or to a mixturecomprising phosphoric acid and nitric acid; and transporting an electriccurrent between an electrode and the gold and/or silver of the scrapmaterial such that at least a portion of the gold and/or silver isremoved from the scrap material.

Some embodiments are related to methods of recovering gold. In someembodiments, the method of recovering gold comprises combining water anda gold-containing solution comprising dissolved gold, nitric acid andsulfuric acid, and/or nitric acid and phosphoric acid to form a mixture,such that solid gold is precipitated within the mixture.

Certain embodiments are related to methods of recovering silver. Incertain embodiments, the method comprises exposing a scrap materialcomprising the silver and cadmium oxide to a mixture of sulfuric acidand nitric acid and/or to a mixture of phosphoric acid and nitric acidsuch that the silver is dissolved by the mixture.

In some embodiments, the method of recovering silver comprises exposinga scrap material comprising the silver and tungsten to a mixture ofsulfuric acid and nitric acid and/or to a mixture of phosphoric acid andnitric acid such that the silver is dissolved by the mixture.

Certain embodiments relate to systems for the recovery of gold and/orsilver from scrap material. In some embodiments, the system comprises arotatable container positioned within a vessel configured to contain aliquid having a pH of less than about 2; and an electrically conductivepathway configured such that, when the scrap material is containedwithin the rotatable container, the electrically conductive pathwayremains in electrical communication with the scrap material when thecontainer is rotated.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1 is, according to one set of embodiments, a cross-sectionalschematic illustration of a system in which scrap items are located onmultiple racks immersed in a vessel containing a leaching solution,which can be recirculated;

FIG. 2 is a cross-sectional schematic illustration of a system in whichscrap items are located within a basket or other container immersed in avessel containing a leaching solution (which can be recirculated),according to certain embodiments;

FIG. 3 is a schematic cross-sectional illustration of a system in whichscrap items are located within a rotatable container within a vesselcontaining leaching solution, according to some embodiments;

FIG. 4 is, according to one set of embodiments, a schematic illustrationof a system in which silver metal is recovered from scrap material;

FIG. 5 is a schematic illustration of a process in which silver and goldare recovered from scrap material, according to certain embodiments;

FIGS. 6A-6B are schematic illustrations showing the dissolution of acoating material, according to certain embodiments;

FIGS. 7-9 are schematic illustrations of processing systems comprising arotatable container, according to some embodiments;

FIG. 10 is a schematic illustration showing an exemplary process bywhich gold is precipitated from a gold-containing solution using water;and

FIG. 11 is a schematic illustration of an exemplary cementation processin which scrap copper is used to produce silver metal from asilver-containing solution.

DETAILED DESCRIPTION

The recovery of gold and/or silver from gold and/or silver containingscrap is generally described. According to certain embodiments, the goldand/or silver can be recovered selectively, such that gold and/or silveris at least partially separated from non-silver and/or non-gold materialwithin the scrap. According to certain embodiments, gold and/or silvermay be recovered from scrap material using mixtures of acids. In someembodiments, the mixture comprises nitric acid and at least onesupplemental acid. For example, in some embodiments, mixtures comprisingsulfuric acid and nitric acid may be used to recover gold and/or silver.In certain embodiments, mixtures comprising phosphoric acid and nitricacid may be used to recover gold and/or silver. In some embodiments, theamount of nitric acid within the mixture may be relatively smallcompared to the amount of sulfuric acid or phosphoric acid within themixture. According to some embodiments, highly concentrated acids may beused in the recovery process (e.g., such that the leaching solutioncontains a relatively small amount of water). In some embodiments, therecovery of gold and/or silver using the acid mixtures described hereincan be enhanced by transporting an electric current between an electrodeand the gold and/or silver of the scrap material. Certain embodimentsrelate to the use of acid mixtures to recover silver from particulartypes of scrap materials. For example, some embodiments relate to theuse of acid mixtures to recover silver from scrap material comprisingsilver metal and cadmium oxide and/or scrap material comprising silvermetal and tungsten metal.

Certain of the processes involve the separation and/or recovery of goldand/or silver in their metallic form(s) from any type of gold and/orsilver containing scrap items (e.g., plated and/or filled scrap items).According to certain embodiments, gold and/or silver can be selectivelyrecovered from the surface of base metals and base metal alloys. Thegold and/or silver within the scrap material can be in any suitableform. For example, in some embodiments, the gold and/or silver may be inthe form of a plating or filling (e.g., which are collectively referredto as a “coating” everywhere below). Examples of such scrap items fromwhich gold and/or silver may be recovered include, but are not limitedto, gold and/or silver filled jewelry, gold and/or silver plated wires,silver/cadmium oxide filled copper, and gold plated electroniccomponents (e.g., CPUs). The invention is not limited to the recovery ofgold and/or silver from the surface of scrap materials, and in someembodiments, gold and/or silver can be also recovered from scrap itemscontaining gold and/or silver metal within the bulk of the article(e.g., at a depth of at least 5% of the maximum cross-sectional diameterof the article). Recovery of gold and/or silver from the bulk can beachieved, for example, by exposing the scrap to the action of theleaching solutions after the scrap items have been reduced to powder.Non-limiting examples of such scrap materials include silver-tungstenpellets and ground electronic chips.

In some embodiments, silver may be recovered from silver-containingscrap items that are substantially free of gold (e.g., including gold inan amount of less than 2 wt %). In certain embodiments, gold may berecovered from gold-containing scrap items that are substantially freeof silver (e.g., including silver in an amount of less than 2 wt %).According to some embodiments, gold and silver may be recovered fromscrap items that contain both gold and silver.

In some embodiments, gold and/or silver can be recovered from a scrapmaterial by exposing the scrap material comprising the gold and/orsilver (and, in some embodiments, at least one base metal) to a mixturecomprising acids. For example, in some embodiments, the scrap materialis exposed to a mixture comprising nitric acid and at least onesupplemental acid. In some such embodiments, the amount of nitric acidwithin the mixture is less than or equal to about 10 wt %, less than orequal to about 9 wt %, less than or equal to about 8 wt %, less than orequal to about 7 wt %, less than or equal to about 6 wt %, less than orequal to about 5 wt %, less than or equal to about 4 wt %, less than orequal to about 3 wt %, less than or equal to about 2 wt %, less than orequal to about 1 wt %, or less. In some embodiments, the amount ofnitric acid within the mixture is as little as about 4 wt %, as littleas about 3 wt %, at little as about 2 wt %, as little as about 1 wt %,as little as about 0.5 wt %, as little as about 0.1 wt %, or less. Insome embodiments, the amount of nitric acid within the mixture is aslittle as about 0.07 wt %, as little as about 0.05 wt %, at little asabout 0.02 wt %, as little as about 0.01 wt %, or less.

A variety of acids can be used as the supplemental acid. In someembodiments, the supplemental acid(s) is capable of forming an insolublesalt with gold and/or silver within the mixture. For example, in someembodiments, phosphoric acid and/or sulfuric acid can be used incombination with the nitric acid. In some embodiments, gold and/orsilver can be recovered from a scrap material by exposing the scrapmaterial comprising the gold and/or silver (and, in some embodiments, atleast one base metal) to a mixture comprising sulfuric acid and nitricacid. In some embodiments, gold and/or silver can be recovered from ascrap material by exposing the scrap material comprising the gold and/orsilver (and, in some embodiments, at least one base metal) to a mixturecomprising phosphoric acid and nitric acid. In certain embodiments, goldand/or silver can be recovered from a scrap material by exposing thescrap material comprising the gold and/or silver (and, in someembodiments, at least one base metal) to a mixture comprising phosphoricacid, sulfuric acid, and nitric acid.

The dissolution of gold and/or silver can occur, in some embodiments,while an electric current is transported between an electrode and thegold and/or silver of the scrap material. Without wishing to be bound byany particular theory, it is believed that applying the electric currentin this way can increase the rate at which gold and/or silver aredissolved. In certain embodiments in which electric current istransported between the electrode and the gold and/or silver, the scrapitems can be used as an anode in an electrolytic cell.

In some embodiments, gold and/or silver can be recovered from scrapmaterial containing gold and/or silver and at least one base metal. Insome such embodiments, the gold and/or silver can be recovered (e.g.,via dissolution of the gold and/or silver) without substantiallydissolving the base metal(s) of the scrap material. Base metals aregenerally non-precious metals (e.g., metals that are not gold, silver,platinum, or palladium). Examples of base metals include, but are notlimited to, iron, nickel, lead, zinc, copper, manganese, tin, antimony,and/or aluminum, as well as alloys comprising same, and combinationsthereof. In some embodiments, the base metals include copper, iron,nickel, lead, and/or zinc, alloys comprising same, and combinationsthereof.

Certain of the methods described herein can be applied for the recoveryof precious metals from types of scrap that are generally difficult toprocess such as, for example, scrap in which impure gold and/or silver(e.g., alloys of gold and/or silver metals with the base metals) areapplied as a coating (e.g., plating, filling) over a substrate, made ofbase metals/base metals alloys. According to certain embodiments, thegold and/or silver can be dissolved and the base metals of the coatingcan be removed, whereby the coating will be removed from the substrate,but it will not lead to any essential dissolution of the base metals ofthe substrate.

In some embodiments, the weight ratio of the supplemental acid withinthe acid mixture to the nitric acid within the mixture is relativelyhigh. For example, in some embodiments, the ratio of the weight of theat least one supplemental acid in the mixture to the weight of thenitric acid in the mixture is at least about 3:1, at least about 4:1, atleast about 5:1, at least about 6:1, at least about 7:1, at least about8:1, at least about 9:1, at least about 10:1, at least about 11:1, atleast about 12:1, at least about 13:1, at least about 14:1, at leastabout 15:1, or at least about 17:1 (and/or, in certain embodiments, upto about 20:1, up to about 50:1, up to about 100:1, or more). When morethan one supplemental acid is present, the ratio of the weight of the atleast one supplemental acid to the weight of the nitric acid iscalculated by adding the weights of all supplemental acids within themixture together, and comparing this number to the weight of the nitricacid within the mixture. In some embodiments, the ratio of the combinedweights of sulfuric acid and phosphoric acid in the mixture to theweight of the nitric acid in the mixture is at least about 3:1, at leastabout 4:1, at least about 5:1, at least about 6:1, at least about 7:1,at least about 8:1, at least about 9:1, at least about 10:1, at leastabout 11:1, at least about 12:1, at least about 13:1, at least about14:1, at least about 15:1, or at least about 17:1 (and/or, in certainembodiments, up to about 20:1, up to about 50:1, up to about 100:1, ormore).

In certain embodiments, the weight ratio of sulfuric acid to nitric acidin the mixture used to recover the gold and/or silver is relativelyhigh. Such relatively high weight ratios can be employed, in certainembodiments, when electrical current is transported between the scrapmaterial and another electrode within the system, which can result inthe electrolytic removal of gold and/or silver from the scrap material.In some such embodiments, the weight ratio of sulfuric acid to nitricacid within the mixture is at least about 12:1, at least about 13:1, atleast about 14:1, at least about 15:1, or at least about 17:1 (and/or,in certain embodiments, up to about 20:1, up to about 50:1, up to about100:1, or more). For example, in one set of embodiments, the leachingsolution includes 90 wt % or more of concentrated sulfuric acid (e.g.,at least 95 wt % sulfuric acid, such as 95-98 wt % sulfuric acid, thebalance of which may be, for example, water) and 10 wt % or less ofconcentrated nitric acid (e.g., at least 68 wt % nitric acid, such as68-70 wt % nitric acid, the balance of which may be, for example,water).

In certain embodiments, the weight ratio of phosphoric acid to nitricacid in the mixture is relatively high. Such relatively high weightratios can be employed, for example, when electrical current istransported between the scrap material and another electrode within thesystem, which can result in the electrolytic removal of gold and/orsilver from the scrap material. For example, in some embodiments, theweight ratio of phosphoric acid to nitric acid within the mixture is atleast about 11:1, at least about 12:1, at least about 13:1, at leastabout 14:1, at least about 15:1, or at least about 17:1 (and/or, incertain embodiments, up to about 20:1, up to about 50:1, up to about100:1, or more). In one set of embodiments, the leaching solutionincludes 90 wt % or more of concentrated phosphoric acid (e.g., 85 wt %phosphoric acid or stronger, the balance of which may be, for example,water) and 10 wt % or less of concentrated nitric acid (e.g., at least68 wt % nitric acid, such as 68-70 wt % nitric acid, the balance ofwhich may be, for example, water).

Not wishing to be bound by any particular theory, it is believed thatthe use of nitric acid in amounts greater than those described abovecan, in certain cases, lead to the formation of NO_(x) compounds in thebath, especially when, for example, the scrap material is being used aspart of an electrolytic cell during removal of gold and/or silver. Inaddition, it is believed that the use of nitric acid in amounts greaterthan those described above can, in certain cases, cause overheating ofthe solution and/or rapid dissolution of base metals of the substrate,which one may desire to avoid.

In certain embodiments, an oxidant can be used (in place of, or inaddition to, the nitric acid) to recover gold and/or silver from thescrap material. For example, some embodiments comprise exposing thescrap material to a fluid comprising an oxidant having the ability todissolve gold and/or silver. In some such embodiments, the amount of theoxidant within the fluid is less than or equal to about 10 wt %, lessthan or equal to about 9 wt %, less than or equal to about 8 wt %, lessthan or equal to about 7 wt %, less than or equal to about 6 wt %, orless than or equal to about 5 wt % (and/or, in some embodiments, aslittle as 4 wt %, as little as 3 wt %, at little as 2 wt %, as little as1 wt %, or less). A variety of such oxidants may be used. In someembodiments, an oxidant with the ability to dissolve gold and/or silveris selected for use. In some embodiments, the oxidant can be in the formof a soluble salt. In certain embodiments, the soluble salt comprises anoxide of manganese, nickel, lead, and/or chromium. One non-limitingexample of an oxidant that may be used is manganese dioxide (MnO₂). Insome embodiments, the mixture comprises the oxidant (e.g., MnO₂) and atleast one supplemental acid (e.g., phosphoric acid and/or sulfuricacid). For example, in some embodiments, mixtures comprising an oxidant(e.g., MnO₂) and sulfuric acid and/or phosphoric acid may be used torecover gold and/or silver. Any of the supplemental acids describedelsewhere herein can, according to certain embodiments, be used incombination with the oxidant. In some embodiments, the oxidant iscapable of producing oxygen by reacting with the supplemental acid(e.g., phosphoric acid and/or sulfuric acid). For example, whenmanganese oxide and sulfuric acid are used, manganese oxide can reactwith sulfuric acid to produce manganese sulfate (MnSO₄), oxygen gas(O₂), and water.

The mixtures described herein that are used to recover gold and/orsilver (which can include for example one or more acids and/or one ormore oxidants, as described above) are also sometimes referred to hereinas “leaching solutions.”

In some embodiments, the amount of water contained in the leachingsolution is relatively low. For example, some embodiments compriseexposing the scrap material to a mixture comprising nitric acid andwater, wherein the amount of water within the mixture is less than orequal to about 17 w % (or less than about 16 wt %, less than about 15 wt%, less than about 14 wt %, less than about 13 wt %, less than about 12wt %, less than about 11 wt %, less than about 10 wt %, less than about9 wt %, less than about 8 wt %, less than about 7 wt %, less than about6 wt %, less than about 5 wt %, less than about 4 wt %, less than about3 wt %, less than about 2 wt %, or less than about 1 wt %). In certainembodiments, as described above, the mixture comprises supplementalacids, such as phosphoric acid and/or sulfuric acid.

In certain embodiments, the mixture comprises sulfuric acid, and theamount of water within the mixture is less than about 8 wt % (or lessthan about 7 wt %, less than about 6 wt %, less than about 5 wt %, lessthan about 4 wt %, less than about 3 wt %, less than about 2 wt %, orless than about 1 wt %).

For example, in some embodiments, the leaching solution contains amixture of sulfuric acid and nitric acid, and the amount of water withinthe mixture is less than about 8 wt % (or less than about 7 wt %, lessthan about 6 wt %, less than about 5 wt %, less than about 4 wt %, lessthan about 3 wt %, less than about 2 wt %, or less than about 1 wt %).In certain embodiments, the leaching solution contains a mixture ofphosphoric acid and nitric acid, and the amount of water within themixture is less than about 17 wt % (or less than about 16 wt %, lessthan about 15 wt %, less than about 14 wt %, less than about 13 wt %,less than about 12 wt %, less than about 11 wt %, less than about 10 wt%, less than about 9 wt %, less than about 8 wt %, less than about 7 wt%, less than about 6 wt %, less than about 5 wt %, less than about 4 wt%, less than about 3 wt %, less than about 2 wt %, or less than about 1wt %).

Without wishing to be bound by any particular theory, it is believedthat the use of highly concentrated acids (as well as relatively lowconcentrations of nitric acid) in the leaching solution allows for theselective dissolution of gold and/or silver from the scrap material. Insome embodiments, the leaching solution contains nitric acid (e.g., inany of the amounts described above) and a relatively large amount of atleast one supplemental acid. In some embodiments, the leaching solutioncontains supplemental acid in an amount of at least about 50 wt %, atleast about 75 wt %, at least about 80 wt %, at least about 85 wt %, atleast about 90 wt %, at least about 95 wt %, at least about 97 wt %, orat least about 98 wt %. When more than one supplemental acid is presentin a leaching solution, the weight percentage of the supplemental acidin the leaching solution is calculated by summing the weight percentagesof each supplemental acid in the leaching solution. For example, if theleaching solution contains 85 wt % sulfuric acid and 5 wt % phosphoricacid, the leaching solution would be said to contain 90 wt %supplemental acids.

In some embodiments, the leaching solution contains sulfuric acid in anamount of at least about 50 wt %, at least about 75 wt %, at least about80 wt %, at least about 85 wt %, at least about 90 wt %, at least about95 wt %, at least about 97 wt %, or at least about 98 wt %. In someembodiments, the leaching solution contains phosphoric acid in an amountof at least about 50 wt %, at least about 75 wt %, at least about 80 wt%, at least about 85 wt %, at least about 90 wt %, at least about 95 wt%, at least about 97 wt %, or at least about 98 wt %.

In some embodiments, the ratio of the amount of gold and/or silverdissolved from the scrap material can be relatively large compared tothe amount of base material dissolved from the scrap material. That isto say, the leaching solution can, in some embodiments, selectivelydissolve gold and/or silver, relative to non-gold and non-silver metals.For example, in some embodiments, the weight ratio of the amount of golddissolved from the scrap material to the amount of base metal(s)dissolved from the scrap material can be at least about 5:1, at leastabout 10:1, at least about 25:1, at least about 50:1, at least about100:1 (and/or, in some embodiments, up to 1000:1, up to 10,000:1, orgreater). In some embodiments, the weight ratio of the amount of silverdissolved from the scrap material to the amount of base metal(s)dissolved from the scrap material can be at least about 5:1, at leastabout 10:1, at least about 25:1, at least about 50:1, at least about100:1 (and/or, in some embodiments, up to 1000:1, up to 10,000:1, orgreater). In some embodiments, the weight ratio of the combined amountof gold and silver dissolved from the scrap material to the amount ofbase metal(s) dissolved from the scrap material can be at least about5:1, at least about 10:1, at least about 25:1, at least about 50:1, atleast about 100:1 (and/or, in some embodiments, up to 1000:1, up to10,000:1, or greater).

In some embodiments, the scrap material comprises a coating comprisingsilver and/or gold over a substrate material comprising at least onebase metal. For example, referring to FIG. 6A, scrap material 600comprises coating 602 (which may contain gold and/or silver, and, insome embodiments, additional metals) and substrate 604 (which caninclude at least one base metal. The gold and/or silver within scrapmaterial 600 can be removed by exposing scrap material 600 to any of theacidic mixtures described herein (e.g., within leaching container 606).In some such embodiments, the mixture comprising sulfuric acid andnitric acid, or comprising phosphoric acid and nitric acid, dissolvesthe gold and/or silver such that the ratio of the mass of the coatingthat is dissolved to the mass of the substrate material that isdissolved is at least about 5:1, at least about 10:1, at least about25:1, at least about 50:1, or at least about 100:1 (and/or, in someembodiments, up to 1000:1, up to 10,000:1, or greater). Referring toFIG. 6B, for example, after scrap material 600 has been processed,according to certain embodiments, the material within coating 602 isdissolved into the leaching solution, and substrate material 604 remainssubstantially undissolved.

In some embodiments, the silver and/or gold containing materialcomprises a plating comprising silver and/or gold and a substratecomprising at least one base metal. In some such embodiments, theleaching solution (e.g., any of the leaching solutions mentioned above)dissolves the gold and/or silver such that the ratio of the mass of theplating that is dissolved to the mass of the substrate material that isdissolved is at least about 5:1, at least about 10:1, at least about25:1, at least about 50:1, or at least about 100:1 (and/or, in someembodiments, up to 1000:1, up to 10,000:1, or greater). In someembodiments, the silver and/or gold containing material comprises afilling comprising silver and/or gold and a substrate comprising atleast one base metal. In some such embodiments, the leaching solution(e.g., any of the leaching solutions mentioned above) dissolves the goldand/or silver such that the ratio of the mass of the filling that isdissolved to the mass of the substrate material that is dissolved is atleast about 5:1, at least about 10:1, at least about 25:1, at leastabout 50:1, or at least about 100:1 (and/or, in some embodiments, up to1000:1, up to 10,000:1, or greater).

Not wishing to be bound by any particular theory, it is believed thatthe selectivity of removal of gold and/or silver relative to the basemetal(s) is achieved due to the use of relatively concentratedsupplemental acids (and concentrations of nitric acid that aresufficiently low to inhibit the formation of NO_(x) compounds) ascomponents of the leaching solution. It is believed that, in certaincases, the base metals may be passivated by the concentrated acids(e.g., via the formation of a base metal oxide), while gold and/orsilver are dissolved in the mixture.

Surprisingly, according to certain embodiments, the base metals whichare alloyed with precious metals, are still dissolved in the leachingsolution, at the same time the base metals of the substrate aresubstantially preserved. For example, in some embodiments, a scrapmaterial containing a silver and cadmium oxide coating over a copperbase material can be exposed to the acid mixture. In some suchembodiments, cadmium and silver are dissolved in the leaching solutionwhile the copper base material is not substantially dissolved. Asanother example, in some karat gold filled items, 40% of Au is alloyedwith 60% of base metals (e.g., Cu, Ni, Fe, etc.) and applied over Cu—Znsubstrate. In certain embodiments, exposing these articles to certain ofthe acid mixtures described herein results in the dissolution of thegold and base metals in the coating, but does not result in thedissolution of the Cu—Zn substrate.

According to certain embodiments in which the scrap material is used aspart of an electrolytic cell, the use of the above mentioned proportionsof the concentrated acids has led to the unexpected discovery that thegold and/or silver as well as the base metals of the coating (e.g., basemetals of the karat gold filling, cadmium of the silver-cadmium oxidecoating, etc.) are dissolved in the leaching solution, but as soon asthe dissolution of the coating is finished, the base metal substratedoes not substantially dissolve in the solution, and the electriccurrent in the cell drops down to substantially zero. In some suchembodiments, the precious metals can be dissolved in the leachingsolution and subsequently recovered, while the base metal substrate isnot substantially damaged by the process (and can therefore be recoveredseparately).

The gold and/or silver that are recovered can be at least partiallyseparated from the scrap material, for example, in metallic form. Insome embodiments, the gold and/or silver that is recovered from theprocess is recovered in high purity (e.g., having a purity of at leastabout 90 wt %, at least about 95 wt %, at least about 99 wt %, or atleast about 99.9 wt %, relative to non-gold and non-silver metals).

According to certain embodiments, gold metal (e.g., high purity goldmetal) can be recovered from the leaching solution, in which gold,optionally silver, and optionally some amount of base metal(s) have beendissolved. In some embodiments, separation of gold from the rest ofmetal(s) occurs by diluting the leaching solution (containingconcentrated acids) with water. In some embodiments, gold can berecovered from a gold-containing solution by combining water and thegold-containing solution to form a mixture, such that solid gold isprecipitated within the mixture. For example, in some embodiments, whenthe solution is mixed with the excess amount of water, silver and/orbase metal(s) remain dissolved, while gold is precipitated (e.g., in theform of pure or substantially pure metal powder). According to someembodiments, the gold may be separated from the dilute leaching solutionand from the rest of metals, for example, via filtration. In someembodiments in which gold removal is performed by passing electricalcurrent through the scrap material, gold can be at least partiallyprecipitated on a cathode through which the current is transported.

In certain embodiments in which silver is present (either in combinationwith gold or on its own), the silver may be recovered, for example, byany of a number of methods, including, but not limited to, addition ofchloride ions, addition of a reducing agent, addition of a base,electrowinning, or cementation. In certain embodiments, silver may berecovered via the cementation of silver with copper. In someembodiments, dissolved silver may be precipitated (e.g., as elementalsilver and/or as a solid silver salt). In some such embodiments, theprecipitated silver may be at least partially separated from theleaching solution and/or other metals. For example, the precipitatedsilver may be filtered from the leaching solution and/or from othermetals. In certain embodiments, a formate salt (e.g. sodium formate),can be used as reducing agent. In some embodiments, a base (e.g. sodiumhydroxide) can be added to the solution. Addition of the base can causeformation of a silver-containing solid (e.g., in the form of silveroxide). In some embodiments, the silver-containing solid (e.g., silveroxide), can be recovered and the silver within the silver-containingsolid can be at least partially extracted (e.g., via smelting). In someembodiments in which silver is at least partially dissolved from scrapmaterial using electrical current, silver can be recovered on a cathodethrough which the electric current is transported.

In some embodiments, silver can be precipitated by adding chloride ions(e.g., by adding hydrochloric acid, sodium chloride, or any othersuitable source of chloride ions). In some such embodiments, the silvercan be precipitated as silver chloride. In some such embodiments, thesolid silver chloride can be at least partially separated from otherdissolved metals that do not form a solid compound with chloride ions(e.g. cadmium). According to certain embodiments, silver chloride can befurther transformed to silver metal using any of a variety of methods(e.g., by addition of hydroxide and/or glucose or by interaction withsodium borohydride).

Silver may also be selectively precipitated as silver metal using othermethods. For example, in some embodiments, silver metal can beprecipitated by adding sodium formate at increased pH (e.g., a pH ofbetween about 1.5 and about 4), which can produce high purity silver. Insome embodiments, silver metal can be precipitated by adding a reducingagent. For example, reducing agents such as hydrazine, hydroquinone,and/or ascorbic acid, can be added to the solution of dissolved silver.

It is noted that, in many conventional operations, the dilution of theconcentrated acids with water is a very dangerous process, in whichNO_(x) compounds and/or large amounts of hot corrosive acid vapor aregenerated. It has been discovered that the dilution of acids inaccordance with certain embodiments described herein can be accomplishedsafely without generation of dangerous vapors and gases, when the loadedleaching solution is slowly added to the excess amount of deionized (DI)water. In certain embodiments, the weight proportion of water toconcentrated acid solution can be 5:1 or higher. In certain embodiments,the weight proportion of water to concentrated acid solution can be 10:1or higher. According to certain embodiments, the concentrated acidsolution is rapidly stirred in the excess volume of water as it isadded. As a result of this operation, gold can be precipitated in theform of a substantially pure metal powder and can be recovered from thesolution, for example, using any type of solid-liquid separationtechnique. Subsequently, scrap copper can be added to the leachingsolution, and silver can be recovered by galvanic displacement(cementation).

Certain of the methods described herein may be used to recover silverfrom scrap material comprising silver and cadmium oxide. For example, insome embodiments, silver is recovered by exposing a scrap materialcomprising silver and cadmium oxide to a mixture comprising sulfuricacid and nitric acid, or to a mixture comprising phosphoric acid andnitric acid, such that the silver is dissolved by the mixture. In someembodiments, the scrap material from which the silver is recoveredcomprises silver and cadmium oxide (e.g., a Ag—CdO alloy) coated orotherwise positioned over a copper base material. Such materials areoften used, for example, in relay contacts, switches, profiles, contacttips, and the like. In some embodiments, the silver-cadmium oxidematerial is dissolved in the leaching solution, the silver is separatelyrecovered, and the copper base is not substantially corroded by theleaching solution. In such a way, the silver-cadmium oxide on copperbase material, which generally cannot be easily recycled by conventionalmethods such as smelting (e.g., due to the dangers caused by melting ofcadmium), can be recycled relatively easily.

In some embodiments, silver can be recovered from scrap materialcontaining silver and tungsten. For example, in some embodiments, silveris recovered by exposing a scrap material comprising the silver andtungsten to a mixture comprising sulfuric acid and nitric acid, or to amixture comprising phosphoric acid and nitric acid, such that the silveris dissolved by the mixture. In the past, the recovery of silver fromscrap containing silver and tungsten has generally not been commerciallyfeasible using conventional methods such as smelting because of veryhigh melting temperature of tungsten. In addition, the use ofconventional leaching solutions for dissolving silver (such as, forexample, solutions containing 50 wt % or 50 vol % nitric acid) may leadto the oxidation of tungsten and the formation of tungsten oxide. Inaddition, in such methods large amounts of nitric acid is consumed forthe oxidation of tungsten, and silver is generally not dissolvedentirely. Surprisingly, the leaching solution formulated according tocertain of the embodiments described herein does not lead to theoxidation of tungsten. In addition, in some embodiments, silver isselectively dissolved and recovered from the solution, leaving behindthe tungsten residue.

Certain embodiments relate to inventive systems and apparatus used torecover gold and/or silver from scrap material. In some embodiments, thesystem comprises a rotatable container made of an electricallyinsulating and/or an electrically conductive material positioned withina vessel. The vessel may be, in certain embodiments, configured tocontain a liquid having a pH of less than about 2 (or less than about 1,less than about 0.5, or less than about 0).

In some such embodiments, the system comprises an electricallyconductive pathway, for example, from a source of an electrical currentto the interior of the rotatable container. In some embodiments, theelectrically conductive pathway may be configured such that, when thescrap material is contained within the rotatable container, theelectrically conductive pathway remains in electrical communication withthe scrap material when the container is rotated.

In some embodiments, the electrically conductive pathway can be incontact with an electrically conductive rotatable container, andelectricity can be transported through the pathway, through theelectrically conductive rotatable container, to the scrap material (and,in particular, the gold and/or silver within the scrap material) as therotatable container is rotated. Non-limiting examples of systems inwhich such arrangements are used are shown in FIGS. 8-9.

In some embodiments, the electrically conductive pathway comprises anelectrically conductive lead, such as a metallic lead. The electricallyconductive lead can be inserted into the rotatable container andpositioned such that the electrically conductive lead remains in contactwith the scrap material (and, in some such cases, the gold and/or silverwithin the scrap material) when the container is rotated. Duringoperation, the scrap can be placed in the rotatable container (e.g., acylindrical container such as a barrel), and the electrically conductivelead(s) (which may be made, for example, of titanium, any othercorrosion resistant conductive material, or any other material) can beinserted such that they make contact with the rotatable container and/orthe scrap material within the rotatable container. In some embodiments,the conductive leads may be positioned such that they continuouslyremain in electrical contact with the scrap items (which may movebecause of the rotation of the rotatable container), so thatsubstantially all the scrap items remain in the electrical contact withthe electrically conductive leads. The leads can be connected to thepositive pole of a direct current source, and in such a way the scrapitems serve as an electrode (e.g., an anode). A non-limiting example ofa system in which such arrangements are used is shown in FIG. 7.

In certain embodiments in which an electrical current is used to atleast partially remove the gold and/or silver, the other electrode(e.g., the cathode) can be made of any corrosion resistant conductivematerial, such as e.g. stainless steel, and can be placed inside of theleaching reactor (e.g., close to the rotatable container). The scrapmaterial can be placed in the rotatable container. The rotatablecontainer can then be closed and immersed into the leaching solutioninside the leaching reactor. In some embodiments, as soon as theelectric current drops to substantially zero, the leaching solution canbe pumped out of the reactor, and, in some such embodiments, rinse watercan be pumped into the reactor. The rotatable container can be rotatedwith or without electrical current being applied, so that the scrapitems can be rinsed from the residues of the leaching solution. In someembodiments, the rinse water can then be transported out of the reactor.At this point the cover of the leaching reactor can be lifted and thetreated scrap material can be removed. Advantageously, human contactwith the concentrated acids can be avoided, according to certainembodiments, making the leaching process safe to use.

An exemplary process for the recovery of silver from silver-containingscrap material is now described. The exemplary process comprises threemain parts: leaching, dilution, and cementation. During the leachingstage, the scrap items are treated with the leaching solution, so thatthe silver containing surfaces are in the direct contact with thesolution, and in such manner, silver metal can be selectively dissolvedin the leaching solution and separated from the remaining scrap items,which are made of base metals. Examples of the materials which have beenalready successfully treated according to this process, include silverplated copper wires (e.g., up to 5 wt % of silver, with the balancecopper), silver filled copper-zinc perforated tapes (e.g., up to 25 wt %of silver, with the balance being copper-zinc alloy), silver-cadmiumoxide filled copper plates (e.g., up to 90 wt % silver, up to 25%cadmium oxide filling on pure copper plates), silver and tungstencontaining pellets (40-45 wt % of silver, with the balance beingtungsten and some base metals). In the last case the pellets werereduced to powder before being exposed to the action of the leachingsolution.

According to certain embodiments, the relative amounts of silver andleaching solution can be up to 60 g of silver per 1 liter of solution,for example, when the solution is at about 55° C. According to someembodiments, if larger amounts of silver are dissolved, the whiteprecipitate of silver sulfate can appear. To dissolve the precipitate,more leaching solution can be added, heated and stirred. In someembodiments, the temperature of the solution is from about 55° C. toabout 100° C. (such as from about 55° C. to about 80° C.).

In some embodiments, after a portion of the silver from the scrapmaterial has been dissolved and subsequently precipitated within theleaching solution (e.g., as silver sulfate), additional supplementalacid (e.g., additional sulfuric acid) can be added to the leachingsolution. The addition of supplemental sulfuric acid can be used toreplenish the acid which has been consumed by the formation of silversalt. Thus, in some such embodiments, as the dissolved silverprecipitates, new portions of the undissolved metal in the scrap can bedissolved as sulfuric acid is added. In some such embodiments, theleaching process can be performed over much longer continuous periods oftime than would be possible were additional supplemental acid not addedto the leaching solution In some embodiments, the solid silver salt(e.g., silver sulfate) can be separated from the leaching solution, forexample, via filtration. In some embodiments in which silver sulfate isformed (e.g., when sulfuric acid is used as a supplemental acid), thesilver sulfate can be reacted to form silver oxide. The formation ofsilver oxide from silver sulfate can be achieved, for example, by addinga base (e.g. sodium hydroxide) to the silver sulfate-containing liquid.In some embodiments, the silver oxide can be transformed to silvermetal. The formation of silver metal from silver oxide can be achieved,for example, via smelting.

In some embodiments, the formation of a silver-containing solid (e.g.,silver precipitate such as silver sulfate) in the leaching solution canbe beneficial in separating the silver from other metals. For example,in some embodiments, a coating on the scrap material can contain a basemetal (e.g., in addition to silver). In some such embodiments, the basemetal in the coating can be soluble in the leaching solution and thesilver may be at least partially insoluble in the leaching solution(and, thus, form a precipitate in the leaching solution). In some suchembodiments, the base metal (e.g., cadmium) may remain dissolved in theleaching solution while the silver precipitates. In some suchembodiments, the silver-containing precipitate may be at least partiallyseparated from the base metal (e.g., cadmium), for example, viafiltration or any other suitable solid liquid separation process. Insome such embodiments, the silver can be recovered in substantially pureform.

In some embodiments (e.g., in certain embodiments in which electricalcurrent is transported through the scrap material during the removal ofsilver), silver salt (e.g., silver sulfate) may be initially included inthe leaching solution (e.g., in an amount up to the saturation limit ofthe silver salt). In some embodiments, the voltage that is appliedacross the scrap material and a second electrode is kept relatively low,such as at or lower than about 5V, at or lower than about 2V. In somesuch embodiments, silver will be deposited on the second electrode(which can be the cathode of the electrochemical cell). In some suchembodiments, the silver dissolved from the scrap material replenishesthe silver deposited on the second electrode. In some embodiments, atthe end of the stripping process, some amount of silver remainsdissolved in the leaching solution. In some embodiments, silver whichhas been deplated during the process can be recovered from the secondelectrode.

In certain embodiments (including certain embodiments in whichrelatively large amounts of gold are dissolved), a gold precipitate(e.g., a dark-brown gold precipitate) can appear in the leachingsolution. In some embodiments, the precipitate can be accumulated andseparated from the leaching solution using filtration or any othersuitable liquid-solid separation technique. According to certainembodiments, if no other metals are dissolved in the leaching solutionalong with gold, the stripping process can be operated over a relativelylong continuous period. In some such cases, replenishing the leachingsolution is not needed unless some of the original leaching solutionescapes from the leaching solution container. In some embodiments inwhich other metals (e.g., base metals and/or silver) are dissolvedtogether with gold in the leaching solution, the dilution of theleaching solution and separation of solid gold can be performed beforethe concentration of the other metal dissolved in the leaching solution(e.g., the base metal and/or silver) exceeds its saturation level withinthe leaching solution. In some such embodiments, mixing of solid goldwith an insoluble salt of a non-gold metal can be avoided, which canenhance the degree to which the gold is purified.

In some embodiments, re-circulation of the solution speeds up theprocess. To prevent shadowing, the scrap items can be strategicallyarranged. For example, in some embodiments, the scrap items can beplaced in one layer (without overlapping) on multiple racks. In somesuch embodiments, the leaching solution can be circulated around thescrap items. Such an arrangement is illustrated, for example, in FIG. 1.In FIG. 1, scrap items 101 are located within container 103, which cancontain leaching solution. In some embodiments, trays 102 are used tosupport scrap items 101. In some embodiments, leaching solution can becirculated through container 103, for example, by transporting leachingsolution into inlet 104 and through outlet 105.

In some embodiments, the scrap items can be placed in a container andcan be loaded into another container in the middle of the leachingprocess, such that the items change positions and the shadowed zonesbecome exposed. Such an arrangement is illustrated, for example, in FIG.2. In FIG. 2, scrap items 201 are located within first container 202,which is located within second container 203. In some embodiments, thefirst container can have openings through which leaching solution can betransported (into first container 202 from second container 203 and outof first container 202 to second container 203). In some embodiments,leaching solution can be circulated through container 202 and/orcontainer 203, for example, by transporting leaching solution into inlet204 and through outlet 205.

In some embodiments, the scrap items can be placed into a rotatablecontainer (e.g., a large slow rotating barrel). In some suchembodiments, there is little or no need to circulate the leachingsolution. The rotating container can correspond to, for example, any ofa number of commercially available plating barrels. An example of such aprocess is shown in FIG. 3. In FIG. 3, scrap items 301 are locatedwithin rotatable container 302. Rotatable container 302 can be locatedwithin second container 303. In some embodiments, rotatable container302 can have openings through which leaching solution can be transported(into rotatable container 302 from second container 303 and out ofrotatable container 302 to second container 303). In some embodiments,leaching solution can be circulated through container 302 and/orcontainer 303, for example, by transporting leaching solution into aninlet and/or through an outlet of second container 303. In someembodiments, rotatable container 302 can be lowered into secondcontainer 303 (e.g., by moving rotatable container 302 in the directionof arrow 308). In some embodiments, rotatable container 302 can beraised out of second container 303 (e.g., after dissolving gold and/orsilver from scrap material 301), for example, by moving rotatablecontainer 302 in the direction of arrow 309.

The selection of the type of container in which to place the scrapmaterial may depend upon the appearance and/or other qualities of thetypes of scrap that are being processed.

FIG. 7 is an exemplary schematic illustration of a leaching system inwhich a rotatable container (e.g., a barrel) is used to house the scrapmaterial. In FIG. 7, rotatable container 703 is placed inside leachingcontainer 701, which can be at least partially filled with leachingsolution 702. The rotatable container can be at least partially (e.g.,completely) submerged within the leaching solution in container 701.Scrap material 704 can be loaded into rotatable container 703. In someembodiments, the rotatable container is made of electrically insulatingmaterial. In certain embodiments, anode leads (e.g., titanium rods) canbe positioned within rotatable container 703 such that they remain incontact with scrap material 704. In certain embodiments, theelectrically conductive leads are inserted into the rotatable container(e.g., from one or multiple sides of the rotatable container) and remainloose inside the barrel so that the scrap material remains in contactwith the leads. The leads can be connected to the anode, such that thecontact of the leads with the scrap items (which can be in contact witheach other) make the scrap material work as an anode. Cathode 706 can bepositioned within container 701 (e.g., outside rotatable container 703).Cathode 706 can be positioned, in some embodiments, such that theshortest distance between cathode 706 and rotatable container 703 isless than 1 meter, less than 10 cm, or less than 1 cm. In someembodiments, electrical current source 708 (e.g., a DC source or an ACsource) can be configured such that it makes electrical contact withcathode 706 and the scrap material 704 (e.g., via anode leads 705). Insome such embodiments, an electrical current can be applied across theanode and the cathode, to aid in the dissolution of gold and/or silverfrom the scrap material. In some embodiments, leaching container 701comprises an inlet 707A and/or outlet 707B. Inlet 707A and outlet 707Bcan allow one to circulate leaching solution and/or rinse liquid intoand out of the leaching reactor.

FIG. 8 is another exemplary schematic illustration of a leaching systemin which a rotatable container is used to house the scrap material,according to certain embodiments. In FIG. 8, rotatable container 803 isplaced inside leaching container 801, which can be at least partiallyfilled with leaching solution 802. The rotatable container can be atleast partially (e.g., completely) submerged within the leachingsolution in container 801. Scrap material 804 can be loaded intorotatable container 803. Cathode 805 can be positioned within container801 (e.g., outside rotatable container 803). Cathode 805 can bepositioned, in some embodiments, such that the shortest distance betweencathode 805 and rotatable container 803 is less than 1 meter, less than10 cm, or less than 1 cm. In some embodiments, electrical current source808 (e.g., a DC source or an AC source) can be configured such that itmakes electrical contact with cathode 805 and the scrap material 804. Insome such embodiments, rotatable container 803 can comprise anelectrically conductive material. For example, rotatable container 803can be made of an electrically conductive material (e.g., a corrosionresistant conductive material) and/or rotatable container 803 caninclude an electrically conductive lining/insert (e.g., made ofcorrosion resistant conductive material). In certain embodiments,rotatable container 803 is connected to the anode of the current source,and the rotatable container can itself serve as part of the anode.Current can be transported to the scrap material within the rotatablecontainer via the electrically conductive material from which at least aportion of the rotatable container is made. In some such embodiments, anelectrical current can be applied across the anode and the cathode, toaid in the dissolution of gold and/or silver from the scrap material. Insome embodiments, container 801 comprises an inlet 806A and/or outlet806B. Inlet 806A and outlet 806B can allow one to circulate leachingsolution and/or rinse liquid into and out of the leaching reactor.

FIG. 9 is another exemplary schematic illustration of a leaching systemin which a rotatable container is used to house the scrap material,according to certain embodiments. In FIG. 9, rotatable container 904 isplaced inside container 901, which can be at least partially filled withleaching solution 903. The rotatable container can be at least partially(e.g., completely) submerged within the leaching solution in container901. Scrap material 905 can be loaded into rotatable container 904. Insome such embodiments, container 901 can comprise an electricallyconductive material. For example, container 901 can be made of anelectrically conductive material (e.g., a corrosion resistant conductivematerial) and/or container 901 can include an electrically conductivelining/insert (e.g., made of corrosion resistant conductive material).In some embodiments, container 901 can be connected to the cathode ofthe current source, and container 901 can itself serve as part of thecathode. At the same time, container 901 can contain the leachingsolution. In some embodiments, cathode plates 902 can be positionedwithin container 901 (e.g., outside rotatable container 904). Forexample, cathode plates 902 can be attached to the inner surface ofcontainer 901. Cathode plates 902 can increase the cathode surface area.Cathode plates 902 can be positioned, in some embodiments, such that theshortest distance between cathode plates 902 and rotatable container 904is less than 1 meter, less than 10 cm, or less than 1 cm. In some suchembodiments, rotatable container 904 can comprise an electricallyconductive material. For example, rotatable container 904 can be made ofan electrically conductive material (e.g., a corrosion resistantconductive material) and/or rotatable container 904 can include anelectrically conductive lining/insert (e.g., made of corrosion resistantconductive material). In certain embodiments, rotatable container 904 isconnected to the anode of the current source, and the rotatablecontainer can itself serve as part of the anode. Current can betransported to the scrap material within the rotatable container via theelectrically conductive material from which at least a portion of therotatable container is made. In some embodiments, an electrical currentsource (e.g., a DC source or an AC source) can be configured such thatit makes electrical contact with cathode plates 902 and the scrapmaterial 905. In some such embodiments, an electrical current can beapplied across the anode and the cathode, to aid in the dissolution ofgold and/or silver from the scrap material. In some embodiments,leaching container 901 comprises an inlet 906A and/or outlet 906B. Inlet906A and outlet 906B can allow one to circulate leaching solution and/orrinse liquid into and out of the leaching reactor.

In some embodiments, the rotatable container can include openings (e.g.,perforations) in its wall. By including such openings, leaching fluidmay be transported into and out of (and, in some cases, can becirculated within) the rotatable container during the leaching process.

The leaching solution is considered saturated (loaded with silver) whenthe concentration of silver rises to a threshold value and remainsconstant. In some embodiments, selective silver electrodes can be usedto monitor the concentration of silver in the solution. Additionally,the conductivity of the solution and/or the specific gravity of thesolution can also be used as control parameters, as these parameterswill change very little (or not at all) once the solution has becomesaturated with silver (perhaps changing slightly because of some minorleaching of the base metals). The end of the leaching process can bealso determined by observation at the moment, for example, when thesilver coating is visibly removed. In some embodiments, when silver isaccumulated in the form of silver salt, the solution is considered to besaturated when the concentration of the dissolved base metals ions risesto a threshold value and remains substantially constant. In someembodiments, selective cadmium electrodes can be used to monitor theconcentration of cadmium in the solution. After finishing the leachingprocess, the rotatable container can be immersed in rinse water. Therinsed, deplated items can then be removed from the process.

The leaching solution can contain any of the components described indetail above. For example, in one set of embodiments, the leachingsolution includes 90 wt % of 98 wt % sulfuric acid and 10 wt % of 68 wt% nitric acid. The relatively low amount of water within the leachingsolution can ensure that copper, zinc, and/or other substrate metalswill not dissolve during the dissolution of the silver plating. Thedissolved silver can form silver nitrate, as follows:

Ag+2HNO₃→AgNO₃+NO₂+H₂O  [1]

The silver nitrate can be chemically reacted to form silver sulfate andaccumulated in the solution. Generally, addition of water to thissolution should be avoided, as it is often accompanied by vigorousformation of NO_(x) compounds. Additions of nitric acid can be made ifsome evaporation of the solution has occurred.

Nitrogen dioxide dissolves in water, consuming oxygen from the air, sono gas formation is observed:

4NO₂+2H₂O+O₂→HNO₃  [2]

Silver nitrate can interact with the sulfuric acid, and can beaccumulated in the leaching solution as silver sulfate. The nitric acidcan be liberated to dissolve new portions of silver:

AgNO₃+H₂SO₄=Ag₂SO₄+HNO₃  [3]

As the leaching solution is highly acidic, it can be advantageous,according to certain embodiments, to conduct the process in a closedleaching reactor.

FIG. 4 is a schematic illustration outlining an exemplary sequence ofoperations. In FIG. 4, the leaching solution is prepared by mixing thecomponents in separate leaching solution preparation container 401, inwhich the solution is heated up to temperature. The scrap is positionedwithin a rotating drum, which is inserted into a leaching tank 402. Thecover of the leaching tank is closed and the leaching solution is pumpedfrom tank 401 into the tank 402. According to certain embodiments, thebarrel can then be rotated and the leaching process can start. Thesamples of the leaching solution can be taken for monitoring the silverconcentration, the conductivity, and/or the specific gravity of thesolution. When the leaching process is over, the loaded leachingsolution can be drained/pumped into the next tank 404 for furthertreatment. The scrap can be optionally brought in contact with the freshportion of the leaching solution and exposed to a short period ofleaching (e.g., 2-3 min) to remove small amounts of residual silver.After finishing this process, the slightly loaded leaching solution canbe pumped out of the tank and kept for the next leaching cycle. Withoutopening the tank, the tank can be filled with DI rinse water, and thebarrel can make several turns to rinse the scrap items. After rinsing,the rinse water can be pumped out of the tank and sent directly to thecementation of the rinse water 405. After rinsing, the tank can befilled with the new portion of DI rinse water, which can be stored inthe rinse water tank 403. The cover of the leaching tank can then beopened, and the rinsed scrap items can be unloaded. Subsequently, a newportion of scrap can be loaded in the drum, inserted in the tank, andcovered. The slightly loaded leaching solution can then be brought incontact with the scrap. When the leaching is over, the material can berinsed with the rinse water from the tank 403, which can then be sent tothe cementation process. The tank can then be rinsed with fresh DI waterfor the second time, which can then be stored in the rinse water tank,etc.

In general, the leaching time depends on the thickness of the plating,and can take between 5 min and 60 min. The second leaching step can, insome embodiments, be 2-3 min. In some embodiments, each rinse processtakes 2-3 min.

The loaded leaching solution can be diluted with DI water as the nextstep of the process. In certain embodiments, the proportion of DI waterto leaching solution can be from about 5:1 to about 20:1. In someembodiments, the proportion of DI water to leaching solution can be fromabout 10:1 to about 20:1. The dilution can be accomplished verycarefully by adding small portions of the loaded leaching solution tothe full amount of water. In some embodiments, the combined fluids areimmediately and vigorously stirred. By proceeding in such a manner,formation of NO_(x) compounds can be avoided. According to certainembodiments, the resulting solution is warm (e.g., about 40° C.) with anacidic pH (e.g., pH=−0.05). In some embodiments, cold water and/or icemay be used to reduce the temperature of the fluids.

After dilution, the leaching solution can be forwarded to a cementationreactor. In certain embodiments, urea can be added to the dilutedleaching solution. The urea may be added, according to some embodiments,to neutralize the unused nitric acid, which may remain after theleaching process. In certain embodiments, the urea is added to thediluted leaching solution until fuzzing substantially stops. In thecementation reactor, the leaching solution can be left in contact withscrap pieces of pure copper.

Cementation with copper can be an advantageous way to recover metallicsilver, according to certain embodiments, because it is simple toimplement and provides a good selectivity for silver. Also, scrap copperis generally easily available. The overall reaction is:

2Ag⁺+Cu⁰→2Ag⁰+Cu²⁺  [4]

Stirring the solution can accelerate the process. Generally, silversponge will appear in the solution, and some copper will be dissolved.In some embodiments, the pieces of copper are generally large enough toassure easy separation of the sponge silver. According to Equation 4,each part of the recovered silver requires about 0.3 parts of copper (byweight). Although in some such embodiments some dissolution of copperoccurs because of the presence of the nitric acid, it has beenestablished experimentally that the consumption of copper is usuallyabout ⅓ of the weight of the recovered silver. The cementation isgenerally complete when the concentration of silver in the solutiondrops to substantially zero (which can be determined, for example, usinga chloride ion test). The silver sponge can be filtered out of thesolution, rinsed with DI water to remove the contamination of copperions, and can be melted into bars. Cementation with copper willgenerally produce relatively pure silver of 98-99 wt %.

FIG. 11 is a schematic illustration of an exemplary cementation processin which scrap copper is used to produce silver metal from asilver-containing solution.

The same cementation process can also be used to recover dissolvedsilver from the spent rinse water. The amount of copper in the spentrinse water is generally less than the amount of copper in the liquidfrom the main process, as the rinse water is less acidic.

Alternatively, the rinse water can be pumped through a packed columncontaining ionic exchange resins, in order to capture the residualsilver ions.

After the cementation, the solution will generally contain dissolvedcopper and some minor base metals if they were initially present in thescrap material. The spent solution can be forwarded to the waste watertreatment. If it is preferred to recover copper in the metallic form, itmay be cemented out using any other scrap metal such as iron, zinc,aluminum, and the like.

In certain embodiments, silver can be recovered from scrap materialcomprising silver and cadmium oxide (CdO). If the material contains aAg—CdO coating, which was relatively thick in the treated samples, itmay be advantageous to increase the reaction speed by using anodicstripping. The scrap material can be used as an anode, and stainlesssteel rods (or other conductive materials) can be used as a cathode. Theelectrodes can be connected to a power supply, and an electric currentcan be applied. The current will drop to at or near zero when thedissolution of the coating is over. It is favorable, in some cases, touse moderate amperage to avoid overheating of the solution, which maylead to the formation of NO_(x) compounds. The set-up configurationsshown in FIGS. 1-3 can be all used in this process. In such cases, therack, the container/reactor, and/or the rotatable container can beconnected to the power supply and used as an anode. The rotatablecontainer (e.g., in the form of a plating barrel, which is normally usedfor plating operation) can be used as the electrode in this process, asthe anodic bar inside the barrel can be configured to remain inelectrical contact with the treated material.

In some such embodiments, the loaded leaching solution will containsilver as well as the dissolved cadmium. Cementation with copper canselectively remove silver from the solution. In some cases in which itis desirable to recover cadmium before the spent solution is sent to thewaste water treatment, a second cementation operation can be performedin which the cadmium and/or copper are cemented and recovered in theirmetallic form. Cadmium and/or copper can be cemented on e.g. scrap zinc,iron and/or aluminum. As an example, cementation of cadmium with Zn canoccur as follows:

Cd²⁺+Zn⁰→Cd⁰+Zn²⁺  [5a]

As another example, cementation of copper with Zn can occur as follows:

Cu²⁺+Zn⁰→Cu⁰+Zn²⁺  [5b]

In some instances in which copper and cadmium are both present, thecopper can be displaced from the solution before cadmium. Thus, thisprocess can be used for separate recovery of copper and cadmium. In somesuch embodiments, the concentration of copper in solution is monitored(e.g., continuously monitored). In some such embodiments, once theconcentration of copper drops to substantially zero, the copper metalcan be separated from the solution by any suitable solid-liquidseparation technique. In some such embodiments, the solution continuesto remain in contact with the zinc, and cadmium can then be cemented out(and, according to certain embodiments, recovered).

The cementation of copper and/or cadmium can be accomplished, forexample, in an agitated vessel, with a removable zinc liner, on piecesof scrap zinc, or via any other suitable method.

An exemplary process for the recovery of gold and silver from gold andsilver-containing scrap material is now described. In this exemplaryprocess, gold and silver are recovered separately. There are two maindifferences between this process and the process described above forsilver recovery. First, anodic stripping is employed. Second, afterdilution, the solution is filtered to remove fine particles of the solidgold metal. Accordingly, the exemplary process described below includethe following four main steps: anodic dissolution-dilution-filtration ofgold powder-cementation of silver.

Items which have been treated in experiments using this method includekarat gold filled scrap, gold and silver filled scrap, and gold platedcopper wires. FIG. 5 is a schematic illustration of an exemplary processused to treat gold and silver containing scrap. The process is similarto that described above for the silver-cadmium containing scrap, withsome additions. The gold and silver-containing scrap material can be puton racks, in baskets, or in a rotatable container (e.g., a barrel) 502,which can be used as an anode. Stainless steel bars (or other conductivematerials) can be used as a cathode. Electrical current can be appliedacross the anode and the cathode to aid in the dissolution of the goldand silver. Generally, the electrical current drops to at or near zeroonce the process of dissolving the silver and gold is complete. Gold,silver, and minor amounts of base metals can be dissolved in thesolution. The moment when the solution can be considered saturated canbe determined by measuring the conductivity and/or the specific gravityof the solution. The saturation can also be detected by observing theremoval of gold and silver from the stripped scrap material.

The leaching solution used in the silver process described above is alsocapable of oxidizing and dissolving gold. In cases in which gold isdissolved, the speed with which the process is performed can beincreased if electrical force (anodic stripping) is used. Thus, thestrong acid liquids described elsewhere herein can be used to dissolvesilver, gold, and/or combinations of silver and gold from scrapmaterials.

Not wishing to be bound by any particular theory, dissolution of thegold and silver may take place as follows. When the stripping solutioncontains nitric acid and sulfuric acid, nitric acid acts as a base onthe sulfuric acid, forming nitronium cations NO₂ ⁺

HNO₃+2H₂SO₄→NO₂ ⁺+H₃O⁺+2HSO₄ ⁻  [6]

The formation of a nitronium ion NO₂ ⁺ can occur through the addition ofa proton to HNO₃, as follows:

The nitronium cations can oxidize gold:

Au⁰+3NO₂ ⁺→Au³⁺+3NO₂  [8]

Nitric acid exists in the solution in equilibrium, according to:

2HNO₃←→NO₂ ⁺+NO₃ ⁻+H₂O  [9]

When water is added in excess, Equation 9 is shifted toward the nitricacid side, NO₂ ⁺ ion is no longer present in the solution, and goldprecipitates in metallic form.

Accordingly, in certain embodiments, the second step of the process isthe dilution of the solution in Equation 4, which can be accomplished asdescribed above for the silver process. During the dilution, very finegold dust can be formed in the solution. The gold dust can have theappearance of fine black/deep purple particles. In general,substantially all of the dissolved gold can be precipitated out ofsolution, such that the diluted solution does not contain anysubstantial amounts of additional dissolved gold. FIG. 10 is a schematicillustration showing an exemplary process by which gold is precipitatedfrom a gold-containing solution using water. The gold powder can befiltered out of the solution and recovered. The filtered gold can bewashed with DI water and melted into a bar.

The remaining leaching solution contains silver and, in some cases,minor amounts of some base metals. For selective recovery of silver,cementation with copper 505 can be used (which can be the same processas described above for the silver recovery process). The resultingsilver sponge can be rinsed and melted. The spent leaching solution canbe discarded and/or treated as waste water.

The spent rinse water in container 506 can contain fine gold powder,which can be filtered out to recover gold. The remaining rinse watercontains some dissolved silver, which can be recovered by cementationwith pure copper in vessel 507, as described above for the silverrecovery process.

The following examples are intended to illustrate certain embodiments ofthe present invention, but do not exemplify the full scope of theinvention.

Example 1

This example describes the removal of silver from scrap materialcontaining thick silver plating on a tape made of a copper-zinc alloy.Both sides of the tape were analyzed with a SPECTROSCOUT XRF (x-rayfluorescence) Analyzer (AMETEK). The silver-plated side of the tape wasfound to be covered by almost pure silver, having the followingcomposition:

Ag=99.5±0.5 wt %,

Zn=0.41±0.05 wt %,

Cu=0.043±0.032 wt %.

The non-plated side was identified as a copper-zinc alloy with thefollowing composition:

Cu=62.2±0.4 wt %,

Zn=37.8±0.4 wt %.

A plate having a length of 5 inches was cut off. One side of it (2.2inches long) was immersed into a leaching solution. The leachingsolution was composed of 85 ml of concentrated sulfuric acid (certified95.0 to 98.0 wt %, by Fisher Sci.) and 15 ml of concentrated nitric acid(certified 68.0 to 70.0 wt %, by Fisher Sci.). Considering the highestpossible concentration of the concentrated nitric acid (70.0 wt %) andits density (1.41 g/mL), as well as the highest concentration ofsulfuric acid (98.0 wt %) and its density (1.84 g/mL), the concentrationof pure nitric acid in this solution was equal to 8.34 wt %, theconcentration of pure sulfuric acid was 86.33 wt %, and theconcentration of water was 5.34 wt %.

The solution was heated to 60° C. and stirred with a magnetic stirrer.The silver plated sample was left in the stripping solution for 20minutes, and the color of the sample on the treated part changed toyellow, which was the color of the non-plated, copper-zinc alloy side ofthe tape. The stripping solution did not change in color, but became alittle bit more opaque. There was no blue color, which would signaldissolution of copper in the leaching solution (which would beundesirable). The experiment was continued for an additional 5 minutes,and the sample was removed from the solution. The sample was thenimmersed in deionized (DI) water, rinsed, and dried. The stripped sidewas analyzed by XRF and showed the following percentage of metals:

Cu=61.6±0.3 wt %,

Zn=38.3±0.3 wt %.

Silver was not detected. There were no visual signs of corrosion ofcopper-zinc alloy. A volume of DI water that was 10 times larger thanthe leaching solution was prepared in a beaker and stirred by a magneticstirrer at 300 rpm. The leaching solution containing the dissolvedsilver was carefully added to the DI water in small portions, and theresulting solution was vigorously stirred after addition of each smallportion. No formation of brown NO_(x) was observed. As soon as thedilution was over, urea was added to the diluted solution until thefizzing was no longer observed. Scrap copper metal was immersed into thesolution, and cementation of silver started substantially immediately.After two hours, a chloride ion test showed only light opacity, meaningthat almost all silver ions were displaced from the solution. Thesolution was left overnight so that the cementation process could becompleted. Subsequently, powdered silver metal was filtered out of thesolution, dried, mixed with borax, and melted into a nugget in an ashingfurnace AF1 (Vecstar) at 1050° C. The weight of the nugget was 1.283 g.For comparison, a plate of the same length as the length of the strippedsample was cut from the original sample material and weighed (having aweight of 4.801 g). Assuming the sample had the same density of plating,the weight fraction of the recovered silver would be 26.7 wt %, whichrepresents a very heavy plating. Subsequently, the stripped part of theplate was cut off and completely dissolved in 50 vol % nitric acidsolution in order to detect any unstripped silver values; the resultingsolution was analyzed by ICP-OES (SPECTRO ARCOS EOP, AMETEK)(inductively coupled plasma mass spectrometry). The amount of silvermeasured in the solution was 0.748 mg, which represents only 0.06 wt %of the recovered silver nugget's weight, meaning that substantially allof the silver was deplated and recovered in the process.

To calculate the silver content in this type of material, another sampleweighing 2.382 g was prepared and completely dissolved in the leachingsolution, containing 50% by volume of concentrated nitric acid and 50%DI water. The sample of the solution was analyzed by ICP and resulted in0.594 g of dissolved silver in 41 ml of the solution, or in 24.94 wt %of silver content in the sample, which is close to the stripped value of26.7%. This demonstrated that the amount of silver stripped from thesample using the method described in this example corresponds tosubstantially the entire amount of silver in the original sample.

For comparison, the experiment was repeated with the same sample size ofthe same material; the only difference was that the leaching solutionwas composed of 225 ml of concentrated sulfuric acid and 75 ml ofconcentrated nitric acid. Considering the highest possible concentrationof the concentrated nitric acid (70.0 wt %) and its density (1.42 g/mL),as well as the highest concentration of sulfuric acid (98.0 wt %) andits density (1.84 g/mL), the concentration of the pure nitric solutionin this solution was equal to 14.24 wt %, the concentration of puresulfuric acid was equal to 78.06 wt %, and the concentration of waterwas 7.70 wt %.

As soon as the sample was immersed in the leaching solution, a largeamount of brown NO_(x) was generated, and the color of the solutionchanged to blue, which is a characteristic of the formation of copperions. This demonstrated that elevated concentrations of nitric acid inthe leaching solution leads to extensive dissolution of the base metalsubstrate, an undesirable outcome for the selective recovery of preciousmetals.

Example 2

This example demonstrates the removal of gold from gold filled wire. Twopieces of gold filled wire were prepared, weighed, and their surface wasanalyzed by XRF. The mass of the first wire was 2.163 g, and thefollowing metals were detected on its surface:

Au=95.6±0.6 wt %,

Fe=1.74±0.11 wt %,

Cu=1.26±0.06 wt %,

Co=0.81±0.07 wt %,

Ni=0.56±0.08 wt %.

The second piece weighed 3.279 g and had the plating composed of

Au=81.0±0.6 wt %,

Cu=13.9±0.2 wt %,

Ni=4.80±0.11 wt %,

Fe=0.24±0.08 wt %.

The samples were attached, one after another, to the positive pole of adirect current regulated power supply (BK Precision 1621A) and they wereused as an anode. A stainless steel electrode weighing 196.4 g withdimensions of 26.2×3.1×0.3 cm was connected to the negative pole andserved as a cathode. Both electrodes were installed vertically in abeaker and clipped with plastic clippers to the walls. A 1.5-cm stirringbar was rotated in the center of the beaker at 200 rpm. The beakercontained leaching solution composed of 95 ml of concentrated sulfuricacid and 5 ml of concentrated nitric acid. Considering the highestpossible concentration of the concentrated nitric acid (70.0 wt %) andits density (1.41 g/mL), as well as the highest concentration ofsulfuric acid (98.0 wt %) and its density (1.84 g/mL), the leachingsolution contained:

sulfuric acid—94.2 wt %,

nitric acid—2.71 wt %,

and water—3.09 wt %.

During the first leach, the gold filled wires were not immersed into thesolution completely, as the part of the wire which was attached to analligator clip, was above the surface of the solution. When one side ofthe sample was deplated, the wire was turned and attached by the treatedside, and the non-treated side was immersed in the solution. The initialelectrical current parameters for the first sample were 1.2 A and 5.8Vin a constant voltage mode. During the experiment the current wasdecreasing, and the solution gradually turned yellow. At the end of thestripping process the current dropped down to 0.01 A, and the experimentwas stopped. The total process took 4.2 minutes. The sample was removedfrom the solution, washed with DI water, dried, and its surface wasanalysed by XRF in order to detect unstripped precious metals, ifpresent. The analysis gave the following results:

Cu=99.2±0.4 wt %,

Fe=0.50±0.08 wt %,

Sb=0.23±0.11 wt %,

Co=0.039±0.030 wt %.

Similar treatment was done with the second wire, and its deplatedsurface had the following composition:

Cu=99.1±0.2 wt %,

Sb=0.75±0.09 wt %.

The results confirm that all the gold was stripped from the surface ofthe samples. Subsequently, the stripping solution was mixed with anamount of DI water that was 10 times larger than the leaching solution;the leaching solution was added to water by small portions and vigorousstirring. Black powder particles appeared in the diluted solution.Initially very fine, the particles agglomerated and became larger withtime. The solution decanted fast and it was filtered using vacuumfiltration unit and cellulose filter paper with the pore size of lessthan 0.45 micrometers. The recovered black solid was mixed with boraxand smelted in a furnace at 1150° C. The resulting gold nugget's weightwas 0.1609 g, which corresponds to 2.96 wt % of the untreated sample.The surface of the nugget was analyzed by XRF, showing the followingelemental composition:

Au=94.7±0.3 wt %,

Cu=3.30±0.04 wt %,

Ni=2.0±0.04 wt %.

which is evidence of a high purity of gold.

Example 3

This example describes experiments in which leaching solutions havingvarying concentrations of nitric acid were used to remove silver fromsilver-plated ware. A piece of a flat silver plated ware was cut intoseveral pieces of approximatively the same size, and each piece wasweighed and analyzed by XRF. The typical elemental composition of thismaterial was:

Cu=61.7±0.5 wt %,

Zn=30.4±0.4 wt %,

Ag=4.90±0.30 wt %,

Ni=3.00±0.13 wt %.

A piece of stainless steel having dimensions of 15.5×1.1×0.3 cm was usedas a cathode and each piece of silver plated ware was used as an anode.Both the cathode and the anode were connected to the corresponding poleof direct current regulated power supply (BK Precision 1621A). Leachingsolutions were prepared, which were composed of 75 ml of concentratedsulfuric acid and volumes of nitric acid that varied from 0.1 ml in thefirst experiment to 20 ml in the last experiment (See Table 1 below).The leaching solution was poured in a 200 ml beaker and stirred by amagnetic stirrer. The solution was heated to 60° C. and this temperaturewas kept constant during the experiments.

At the beginning of each experiment a maximum current (0.09 A) wasestablished in constant voltage mode, and both the starting current andthe starting voltage (2.9V) had the same values for all experiments.Each time the deplating process was stopped as soon as the currentdropped to 0.01 A. The remaining piece of the sample was then removedfrom the solution, washed with DI water, dried, and weighed. Thesample's surface was analyzed by XRF. At the lowest concentration ofnitric acid (0.07 wt %), all the silver was removed from the sample whenthe current dropped down to 0.01 A. The same complete removal of silverwas achieved in all of the following experiments. The time required forthe current to drop to zero was monitored. The surface of the deplateditems was analyzed by XRF and was shown to be a copper-zinc alloy:

Cu=64.9±0.4 wt %,

Zn=35.1±0.4 wt %.

At a concentration of nitric acid equal to 10.87 wt %, intense evolutionof colorless fumes started, and the intensity of the fumes increased at11.88%. At 10.87 wt % of nitric acid, the experiment started at 2.9V and0.09 A, as in all the previous cases, but instead of dropping down, thecurrent increased up to 0.22 A during the first minute, and started todrop down only afterwards. The processing time increased compared to thecases when the concentration of the nitric acid was lower. At 11.88 wt %the phenomenon of current increase repeated; it increased up to 0.22 Aduring the first 1.15 min of treatment, and subsequently decreased. Thedeplating process took 7.23 min, which is 3.5-4.5 times longer than theprocess took at lower concentrations of nitric acid. High currents andlong processing times mean the sudden increase of the consumption ofenergy, which resulted in the increase of the temperature of thesolution up to 90° C., which provoked even higher evolution of fumes.The results of these experiments are presented in Table 1.

TABLE 1 Results for deplating silver plated wire samples sulfuric nitricsulfuric nitric water sample sample acid in acid in acid in acid in inmass mass leach leach leach leach leach before after weight time of sol.sol. sol. sol. sol. leach leach loss treatment (mL) (mL) (wt %) (wt %)(wt %) (g) (g) (%) (min) notes 75.00 0.10 97.90 0.07 2.03 3.738 3.7270.305 1.05 all Ag removed 75.00 0.50 97.50 0.36 2.14 3.732 3.718 0.3621.08 all Ag removed 75.00 2.50 95.56 1.74 2.70 4.326 4.309 0.386 0.50all Ag removed 75.00 9.00 89.75 5.89 4.36 4.202 4.184 0.436 2.03 all Agremoved 75.00 14.00 85.74 8.76 5.50 3.194 3.184 0.326 1.21 all Agremoved 75.00 16.30 84.01 9.99 6.00 4.132 4.110 0.532 2.10 all Agremoved 75.00 18.00 82.78 10.87 6.35 4.489 4.464 0.550 4.36 currentincrease, fumes 75.00 20.00 81.37 11.88 6.75 5.045 5.021 0.470 7.23current increase, fumes

Example 4

This example describes experiments in which leaching solutions havingvarying concentrations of nitric acid were used to remove gold fromgold-plated copper wire. A piece of a gold plated copper wire was cutinto the sections of approximatively similar length and weight, and thepieces were analyzed by XRF before and after each deplating process. Anexample of the XRF analysis of the wire before deplating is presentedbelow:

Au=38.2±0.7 wt %,

Ni=36.5±0.4 wt %,

Cu=24.9±0.3 wt %,

Fe=0.21±0.10 wt %.

The experiment was repeated as described in Example 3. In the firstexperiment, all gold was removed from the wire, confirmed by XRFanalysis. As the nitric acid concentration in the leaching solution wasincreased, all the gold was always removed from the treated samples, butthe percentage weight loss of the samples continuously increased. At anitric acid concentration of 10.87 wt %, intense generation of colorlessfumes was observed, which made experiments using such high levels ofnitric acid difficult to perform. Increasing the concentration of nitricacid up to 11.88 wt % lead to almost triple the percentage weight loss,compared to the percentage weight loss of the first sample.Additionally, the acid fumes became very heavy, the time of treatmentalmost doubled; accordingly, further increase of nitric acidconcentration were stopped. The results of these experiments aresummarized in Table 2.

TABLE 2 Results for deplating gold plated wire samples. sulfuric nitricsulfuric nitric water sample sample acid in acid in acid in acid in inmass mass leach leach leach leach leach before after weight time of sol.sol. sol. sol. sol. leach leach loss treatment (mL) (mL) (wt %) (wt %)(wt %) (g) (g) (%) (min) notes 75.00 0.10 97.90 0.07 2.03 0.239 0.2370.753 1.25 all gold removed 75.00 0.50 97.50 0.36 2.14 0.221 0.220 0.8581.00 all gold removed 75.00 2.50 95.56 1.74 2.70 0.212 0.211 0.707 1.05all gold removed 75.00 5.80 92.52 3.92 3.57 0.242 0.239 1.035 1.50 allgold removed 75.00 9.00 89.75 5.89 4.36 0.266 0.263 1.055 1.25 all goldremoved 75.00 12.50 86.90 7.93 5.17 0.257 0.253 1.401 1.30 all goldremoved 75.00 13.30 86.28 8.37 5.35 0.228 0.224 1.450 1.26 all goldremoved 75.00 14.00 85.74 8.76 5.50 0.215 0.211 1.491 1.50 all goldremoved 75.00 15.00 84.98 9.30 5.72 0.209 0.206 1.434 1.20 all goldremoved 75.00 16.30 84.01 9.99 6.00 0.257 0.254 1.321 1.00 all goldremoved 75.00 18.00 82.78 10.87 6.35 0.239 0.235 1.509 1.23 very strongacid fumes 75.00 20.00 81.37 11.88 6.75 0.223 0.218 2.156 2.09 verystrong acid fumes

Example 5

A piece of a gold plated copper wire similar to those used in Example 4was prepared and weighed (0.6274 g). 60 ml of concentrated sulfuric acidwas heated to 60° C. and a measured quantity of manganese dioxide (0.427g) was added to it by stirring. The solution was stirred for 20 minutes,then removed from heat and left overnight. No gas evolution wasdetected. The next day, the solution had dark rose color and someunreacted/undissolved black powder on the bottom. The solution was usedin an electrolytic process, in which the gold plated wire served as ananode and a piece of stainless steel served as a cathode (the same as inthe Example 4). When the power supply was turned on, a current of 0.12 Aand a voltage of 3.2V was applied to the electrode in constant voltagemode. The current started to diminish, and when it reached 0.04 A, thevoltage was increased to 12.3 A, to increase the current and deplate thematerial faster. Soon afterwards, the current dropped down to 0.01 A.The total processing time was 5.07 minutes. During the electrolyticprocess, intensive evolution of gas was observed, which was much moreintensive than the evolution of gas typical for the deplating proceduresin the mixtures of sulfuric acid and nitric acid. Without wishing to bebound by any particular theory, it is believed that the oxygen,liberated in the reaction of MnO₂ with the hot concentrated sulfuricacid, is responsible for dissolution of gold.

2MnO₂+2H₂SO₄→2MnSO₄+O₂+2H₂O

The deplated wire was removed from the solution, rinsed, dried, weighed(0.6198 g) and analyzed by XRF. The amount of weight that was lostcorresponded to 1.21% of the initial weight of the wire. The results ofthe XRF analysis were:

Cu=96.9±0.5 wt %,

Ni=3.03±0.08 wt %.

This demonstrated that that the gold was removed from the surface of thewire.

Example 6

A sample of silver-tungsten containing pellets was prepared. The pelletshad different silver/tungsten weight proportions, which also variedbetween the surface and the center of the pellets, but in general, thepellets contained about 50 wt % of silver metal and 50 wt % oftungsten-containing material (tungsten metal or tungsten carbide). Theconcentration of silver on the surface of the pellets was generallyhigher or equal to 50 wt %, but in the center of the pallets, theconcentration of silver was as low as 5-8 wt %, the rest being tungsten.If the tested pellets are brought in contact with the leaching solutionwithout preliminary size reduction, the silver containing in the centerof the pellets will be inaccessible for the leaching solution and hence,will be lost. To avoid this situation, the sample of the silver-tungstencontaining material was shredded into particles with sizes less than 2mm. The XRF analysis of a randomly taken sample of the powder gave thefollowing results:

W=59.4±0.8 wt %,

Ag=37.7±0.8 wt %,

Fe=0.52±0.11 wt %,

Cu=0.21±0.11 wt %.

A sample of this powder weighing 125.31 g was mixed with a leachingsolution containing 1250 ml of concentrated sulfuric acid and 250 ml ofconcentrated nitric acid, and was left to react for 1 hour at 60° C.with stirring at 300 rpm. Considering the densities of the acids, theweight percentage of the components in the solution was:

sulfuric acid—84.98 wt %,

nitric acid—9.3 wt %,

and water—5.72 wt %.

The solution took the color of the black powder, which had beenliberated by the dissolution of silver. At the end of the process, thesolution was added to a volume of DI water that was 5 times larger thanthe volume of the leaching solution, adding small portions at a time andusing vigorous stirring. Subsequently, the powder was decanted andseparated from the solution using vacuum filtration on Whatman Grade 52paper filters. The recovered powder was dried and analyzed by XRF anddetermined to be almost pure tungsten:

W=96.1±0.6 wt %,

Ag=0.22±0.08 wt %.

The remaining 3-4% were most likely carbon based (because carbon cannotbe detected by XRF), and the analyzed material would accordingly betungsten carbide. The tungsten material looked like black powder. Theweight of the recovered powder was 62.416 g.

The leaching solution free of solids contained dissolved silver. Anexcess of 10N NaOH solution was added to the leaching solution (mixingsmall portions at a time) causing dark-grey precipitate of silver oxideto appear. Addition of NaOH continued until the chloride-ion test(conducted on a drop of the solution), showed no opaqueness. The silveroxide powder was filtered out of the solution using vacuum filter,dried, mixed with borax, and melted in the furnace at 1050° C. Thepurity of the recovered silver nugget was analyzed by XRF:

Ag=99.8±0.2 wt %,

Zn=0.11±0.05 wt %,

Cu=0.08±0.042 wt %.

Its weight was 62.040 g. The results of the experiments are presented inTable 3.

TABLE 3 Results for silver recovery from silver-tungsten containingsamples. Mass of the initial sample, g 125.32 Mass of recovered tungstencarbide, g (wt %) 62.41 (49.8%) Mass of recovered silver nugget, g (wt%) 62.04 (49.5%) Calculated mass of base metals and/or losses, g (wt %)0.87 (0.7%)

For comparison, the same amount of powdered pellets was mixed with asolution containing 50% by volume of concentrated nitric acid, with therest being DI water. The solution was left for 3 hours stirred at 300rpm and heated to 60° C. A build-up of brown NO_(x) was observed.Evaporation losses were replenished with the addition of fresh leachingsolution during the experiment. The solid fraction changed color tocanary green, which is a characteristic color of tungsten trioxide(WO₃). After 3 hours, a small sample of the solid fraction was removedfrom the beaker, rinsed with DI water, dried, and analyzed by XRF. Thesample contained 19 wt % silver. The leaching of the sample continuedfor additional 3 hours. Subsequently, the solution was diluted with 2volumes of DI water and filtered using a vacuum filter. The canary greenpowder was recovered, rinsed with DI water, dried, and analyzed by XRF:

W=90.0±1.2 wt %,

Ag=4.82±0.38 wt %,

Fe=1.86±0.19 wt %,

Cu=1.00±0.16 wt %,

Ni=0.18±0.18 wt %.

This analysis shows relatively large amount of the residualnon-extracted silver.

Example 7

A sample containing silver and cadmium oxide alloy plated on a coppersubstrate was prepared. The sample was 8.1×2.8 cm in size and had twothick silver-cadmium oxide fillings on one side of the copper plate.Both surfaces of the sample were analyzed by XRF. The copper substratewas made of pure copper:

Cu=99.9±0.1 wt %.

The surface of the silver-cadmium oxide plating had the followingcomposition:

Ag=87.6±0.6 wt %,

Cd=12.1±0.3 wt %,

Cu=0.15±0.04 wt %,

Ni=0.044±0.041 wt %.

The inner layers of the plating may have contained up to 12-17 wt % ofcadmium. The weight of the sample was 47.795 g. A leaching solution wasprepared, composed of 304 ml of concentrated sulfuric acid and 16 ml ofconcentrated nitric acid. Considering the densities of the acids, theconcentration of pure nitric acid in this solution was equal to 2.71 wt%, the concentration of pure sulfuric acid was equal to 94.20 wt %, andthe concentration of water was 3.09 wt %.

The leaching solution was poured into a 500 ml beaker; a stainless steelcathode with dimensions of 26.2×3.1×0.3 cm was clipped to the wall ofthe beaker, and the sample plate served as an anode. The cathode and theanode were connected to the corresponding poles of the DC power supplyBK Precision 1794. An initial current at 4.2 A at 9.3V was applied tothe electrodes in constant voltage mode. During the leaching process thecurrent increased to 5.8 A and after some time started to diminish. Thecurrent dropped to 0.02 A in 1 hour and 5 minutes. The copper plate wasdetached and removed from the leaching solution; there was no visualevidence of any remaining unstripped plating. The copper plate wasrinsed with DI water, dried, weighed, and analysed by XRF; the mass ofthe stripped copper plate was 30.924 g; the XRF analysis of the strippedarea showed pure copper. The total amount of leaching solution afterfinishing the process was 310 ml. A sample of the leaching solution wasanalyzed by ICP and following concentrations of the dissolved metalswere obtained:

Ag=42836.1 mg/L,

Cu=824.980 mg/L,

Cd=6031.14 mg/L.

This process allows one to obtain the amounts of metals which weredissolved in the sample of 310 ml: mass of silver—13.27 g, mass ofcadmium—1.87 g and mass of copper—0.82 g. Striping of silver-cadmiumoxide plating led to some dissolution of copper, which resulted in 2.6wt % of the total copper weight.

The leaching solution containing all the dissolved metals was graduallyadded to a volume of DI water 10 times greater than the volume of theleaching solution by continuous stirring. 10N solution of NaOH was addeduntil the pH increased to 4. Subsequently, 5 g of sodium formate wasadded and the solution was stirred at a temperature close to boiling for1 hour. The precipitated silver was separated from the solution byvacuum filtration, it was washed, dried, mixed with borax, and melted ina furnace at 1050° C. A nugget weighing 12.736 g was recovered. Thedifference between the weight of the recovered silver and its calculatedamount based on the ICP measurement was 4.2%. The elemental compositionof the recovered nugget was analyzed by XRF and showed:

Ag=99.3±0.7 wt %,

Ni=0.40±0.07 wt %,

Zn=0.21±0.05 wt %.

For comparison, another sample of the silver-cadmium oxide on coppersubstrate was prepared, and the experiment described above was repeated,but the proportions of the acids in the leaching solution were changed:it contained 270 ml of concentrated sulfuric acid and 90 ml ofconcentrated nitric acid. Considering the highest possible concentrationof the concentrated nitric acid (70.0 wt %) and its density (1.41 g/mL),as well as the highest concentration of sulfuric acid (98.0 wt %) andits density (1.84 g/mL), the concentration of pure nitric acid in thissolution was equal to 14.24 wt %, the concentration of pure sulfuricacid 78.06 wt %, and the concentration of water 7.70 wt %.

The stripping process started as usual, but as the solution started toheat up, a large amount of brown NO_(x) appeared over the beaker, alarge amount of heat was generated, the solution started to boil, and asharp increase of current was observed. The reaction becamenon-controllable and the color of the solution changed to blue, which ischaracteristic for situations in which large amounts of copper aredissolved, an undesirable outcome for the selective recovery of preciousmetals.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,and/or methods, if such features, systems, articles, materials, and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method of recovering gold and/or silver from ascrap material, comprising: exposing the scrap material comprising thegold and/or silver and at least one base metal to a mixture comprisingnitric acid and at least one supplemental acid; and recovering at leasta portion of the gold and/or silver from the scrap material, wherein theamount of nitric acid within the mixture is less than or equal to about10 wt %.
 2. The method of claim 1, wherein the ratio of the weight ofthe at least one supplemental acid to the weight of the nitric acid isat least about 3:1.
 3. The method of any one of claims 1-2, wherein thesupplemental acid comprises phosphoric acid and/or sulfuric acid.
 4. Themethod of any one of claims 1-3, comprising transporting an electriccurrent between an electrode and the gold and/or silver of the scrapmaterial such that at least a portion of the gold and/or silver isremoved from the scrap material.
 5. The method of claim 4, wherein themixture comprises nitric acid and sulfuric acid, and the weight ratio ofsulfuric acid to nitric acid within the mixture is at least about 12:1.6. The method of claim 4, wherein the mixture comprises nitric acid andphosphoric acid, and the weight ratio of phosphoric acid to nitric acidwithin the mixture is at least about 11:1.
 7. A method of recoveringgold and/or silver from a scrap material, comprising: exposing the scrapmaterial comprising the gold and/or silver and at least one base metalto a fluid comprising an oxidant having the ability to dissolve goldand/or silver; and recovering at least a portion of the gold and/orsilver from the scrap material; wherein the amount of the oxidant withinthe fluid is less than or equal to about 10 wt %.
 8. The method of claim7, wherein the fluid comprises at least one supplemental acid.
 9. Themethod of claim 8, wherein the supplemental acid comprises phosphoricacid and/or sulfuric acid.
 10. The method of any one of claims 7-9,wherein the oxidant comprises an oxide of manganese, nickel, lead,and/or chromium.
 11. The method of claim 10, wherein the oxidantcomprises manganese dioxide (MnO₂).
 12. The method of any one of claims1-11, wherein the amount of water in the mixture is less than or equalto about 17 wt %.
 13. The method of claim 12, wherein the mixturecomprises sulfuric acid.
 14. The method of claim 13, wherein the amountof water within the mixture is less than about 8 wt %.
 15. The method ofany one of claims 12-14, wherein the mixture comprises phosphoric acid.16. A method of recovering gold and/or silver from a scrap material,comprising: exposing the scrap material comprising the gold and/orsilver and at least one base metal to a mixture comprising sulfuric acidand nitric acid and/or to a mixture comprising phosphoric acid andnitric acid; and transporting an electric current between an electrodeand the gold and/or silver of the scrap material such that at least aportion of the gold and/or silver is removed from the scrap material.17. A method of recovering gold, comprising combining water and agold-containing solution comprising dissolved gold, nitric acid andsulfuric acid, and/or nitric acid and phosphoric acid to form a mixture,such that solid gold is dissolved within the mixture.
 18. A method ofrecovering silver, comprising exposing a scrap material comprising thesilver and cadmium oxide to a mixture of sulfuric acid and nitric acidand/or to a mixture of phosphoric acid and nitric acid such that thesilver is dissolved by the mixture.
 19. A method of recovering silver,comprising exposing a scrap material comprising the silver and tungstento a mixture of sulfuric acid and nitric acid and/or to a mixture ofphosphoric acid and nitric acid such that the silver is dissolved by themixture.
 20. The method of any one of the preceding claims, wherein thescrap material comprises a coating comprising silver and/or gold over asubstrate material comprising at least one base metal.
 21. The method ofclaim 20, wherein the mixture comprising sulfuric acid and nitric acidor phosphoric acid and nitric acid dissolves the gold and/or silver suchthat the ratio of the mass of the coating that is dissolved to the massof the substrate material that is dissolved is at least about 5:1, atleast about 10:1, at least about 25:1, at least about 50:1, or at leastabout 100:1.
 22. The method of claim 20, wherein the mixture comprisingsulfuric acid and nitric acid or phosphoric acid and nitric aciddissolves the gold and/or silver such that the ratio of the mass of thecoating that is dissolved to the mass of the substrate material that isdissolved is up to about 10,000:1.
 23. The method of any one of claims1-16 and 20, wherein gold and/or silver is dissolved from the scrapmaterial.
 24. The method of any one of claims 17-19 and 21-23,comprising forming a silver-containing solid and/or a gold-containingsolid from the dissolved gold and/or silver.
 25. The method of claim 24,wherein the silver-containing solid comprises silver metal and/or asilver salt.
 26. The method of claim 24, wherein the gold-containingsolid comprises gold metal.
 27. The method of any one of claims 24-26,wherein the silver-containing solid and/or the gold-containing solid isat least partially separated from the mixture.
 28. The method of any oneof claims 1-27, wherein the mixture contains supplemental acid in anamount of at least about 50 wt %.
 29. The method of any one of claims1-28, wherein the mixture contains sulfuric acid in an amount of atleast about 50 wt %.
 30. The method of any one of claims 1-29, whereinthe base metal comprises iron, nickel, lead, zinc, copper, manganese,tin, antimony, and/or aluminum.
 31. The method of claim 30, wherein thebase metal comprises copper, iron, nickel, lead, and/or zinc.
 32. Themethod of claim 31, wherein the base metal comprises copper.
 33. Asystem for the recovery of gold and/or silver from scrap material,comprising: a rotatable container positioned within a vessel configuredto contain a liquid having a pH of less than about 2; and anelectrically conductive pathway configured such that, when the scrapmaterial is contained within the rotatable container, the electricallyconductive pathway remains in electrical communication with the scrapmaterial when the container is rotated.
 34. The system of claim 33,wherein the liquid having the pH of less than about 2 comprises nitricacid and at least one supplemental acid.
 35. The system of claim 34,wherein the liquid having the pH of less than about 2 comprises nitricacid and sulfuric acid.
 36. The system of any one of claims 34-35,wherein the amount of nitric acid within the liquid is less than orequal to about 10 wt %.
 37. The system of any one of claims 35-36,wherein the weight ratio of sulfuric acid to nitric acid within theliquid is at least about 12:1.
 38. The system of any one of claims33-37, wherein the amount of water in the liquid is less than or equalto about 17 wt %.